Magnetic display devices generally utilize a magnetic force to move magnetic particles into a display image. For example, a magnetic display system may include a transparent plastic sheet formed over a surface of honeycomb-shaped cells or cavities, each cavity being filled with a white pigment fluid and magnetic particles. A magnetic field may be applied to the display surface to move the particles into a display image, such as through a rod-like magnetic pen.
Various methods of making a magnetic display are known in the art. For example, a magnetic display may include cells containing spherical-shaped magnetic and non-magnetic particles in fluid sealed between two plates or substrates. In one such system, the particles may include magnetic particles having one color (e.g., black) and non-magnetic particles having another color (e.g., white). A graphic may be initiated on the display by scanning a magnet on or near one or more of the display plates so as to attract one of the two colored particles to the desired surface. While making contact with the surface of the magnetic display using a magnetic pen, the magnetic particle in the area of contact moves to the surface and an image appears.
Various improvements to magnetic displays have been proposed in the art. For example, U.S. Pat. No. 6,097,531, herein incorporated by reference, describes a display system wherein both magnetic fields and electric fields may be applied for addressing, aligning, and moving the elements into place within a display. As another example, U.S. Pat. No. 5,846,630, herein incorporated by reference, describes a magnetic display panel including a dispersion fluid layer composed of a magnetic particle, a dispersion medium, a thickening agent, and a coloring material on a substrate. U.S. Pat. No. 5,411,398, herein incorporated by reference, describes a magnetic display system wherein light-absorptive magnetic particles and light-reflective non-magnetic particles are encapsulated in microcapsules.
As another example, U.S. Pat. No. 5,972,493, incorporated herein by reference, describes gelatin microcapsules for magnetic display, wherein the microcapsules contain a dispersion medium of two solvents and magnetic powder and nonmagnetic powder. Ferromagnetic particles of about 400–800 μm diameter are used. In one embodiment of the magnetic displays described in the '493 Patent, the magnetic and nonmagnetic particles are affinity-treated in order to facilitate particle movement within the display. In one example, a mixture of alumina and silica was used as water-bearing metal oxides. The coating with water-bearing metal oxides is about 1% to 16% by weight of the particle.
Other examples of displays which incorporate magnetic and non-magnetic particles or powders are described in U.S. Pat. Nos. 6,577,433; 6,574,034; 6,529,313 and 6,525,866, each of which is incorporated herein by reference in its entirety. In these patents, a magnetic display device has a number of individual reservoirs, each containing a transparent or colored fluid dispersion medium and two sets of particles, each reservoir being separated within the device by a spacer. In these patents, one embodiment is described wherein within the reservoirs the two sets of particles of the display fluid are encapsulated in a metal oxide shell, preferably a titania shell or silica shell. Encapsulation of particles in a metal oxide shell improves the electrical mobility properties of the particles while retaining the optical or colorant functionality of the underlying particles encapsulated therein.
There are some problems among the magnetic devices comprising both magnetic particles and non-magnetic particles that are available to date. Magnetic and non-magnetic particles used in magnetic displays generally have high density. Thus, magnetic and non-magnetic particles tend to settle within the display device, which results in the loss of image stability over time. Moreover, bare magnetic and non-magnetic particles have a strong tendency to aggregate, and thus require higher magnetic fields for moving. Aggregation decreases resolution of the display. Attempts to produce particles dispersed in a polymer as opposed to an aqueous fluid have been successful for non-magnetic particles to some extent, but magnetic particles, dispersed in a polymer may still have the tendency to aggregate.